Thin film optical recording layers using chalcogenide thin-films and amorphous to crystalline phase transitions have been the subject of many investigations since the early 1970's. The initial interests were focused on "erasable", and therefore reusable, optical recording layers since the amorphous to crystalline transition is, in principle, a reversible process. A low power, relatively long duration laser pulse is used to heat a local spot on the layer to below the melting point for a sufficient length of time to cause the spots to crystallize. These crystalline spots can in turn be heated, by a higher power, shorter duration laser, above the melting point of the crystallized spots to randomize the structure of the spots. The layer is designed such that upon the termination of the laser pulse, the cooling rate of the heated spot is high enough that the randomized structure is frozen to achieve an amorphous state.
Thus by adjusting the laser power and duration, the state of a selected area on the layer can be switched between the amorphous state and the crystalline state to create a pattern of amorphous and crystalline spots which can be used for information storage. Since the phase transition is reversible, the pattern can be erased and replaced with a different recorded pattern. Theoretically, this erase-write cycle can be carried out any number of times.
Very few materials are known for optical recording layers in which the above described write-erase-write cycle is of practical use. No erasable phase-change type optical recording layers have been commercialized.
A good deal of attention has also focused on so-called "write-once" thin film optical recording layers. Write-once simply means that the layers can be recorded upon only once. Such layers cannot be erased and reused for a subsequent recording.
European Patent Application No. 0184452 discloses certain erasable optical recording layers of antimony-indium and antimony-indium-tin alloys. Information recording and erasure are said to be achieved by switching the layers between two diferent crystalline states. The layers are generally prepared in the amorphous states which have to be first converted into one of the two crystalline states before information can be recorded. The crystallization state, achieved by either a bulk heat-treatment or a prolonged laser exposure, is said to have a lower reflectance than the amorphous state. The examples indicate that the materials disclosed therein have a very slow rate of crystallization. This application further teaches that the optical recording layers disclosed therein are unsuitable for use in the amorphous-to-crystalline transition mechanism because of the instability of the amorphous state in general.
The problem is that the prior art has not provided write-once optical recording layers which possess the combination of (a) a crystallization rate less than 1.0 .mu.s, (b) good corrosion resistance, (c) a stable amorphous state and (d) a capability of high rate, high density recordings.
This problem is solved in copending U.S. Ser. No. 194,605 filed May 16, 1988 which is a continuation-in-part of U.S. Ser. No. 014,336 filed Feb. 13, 1987. In that application there is disclosed an alloy of antimony, tin and indium, which alloy is capable of high performance write-once optical recording. The elements of this application do not suffer the environmental corrosion see in chalcogen rich thin films. The rate of crystallization of the optical recording layers is less than 1 .mu.s using practical laser power. The dynamic recording sensitivity at 10 m/s is up to about 6.5 mW, particularly in the range of 3.5 to 6.5 mW. The amorphous state is very stable. Thus, recordings on the thin film are made using the amorphous to crystalline transition mechanism. The layers are capable of high density, high rate recordings having a dynamic carrier-to-noise ratio (CNR) over 60 decibels, particularly in the range of 60 to 65 decibels.
However, while antimony-tin-indium alloys have been found to be useful in erasable and write-once elements, depending on the specific composition of the alloy, a problem has been discovered in the manufacture of the optical recording layers using these alloys. The usual manufacturing process uses a sputtering process. A high voltage is placed between the source of the material to be coated (sometimes referred to as the target) and the substrate to be coated or some other element in the coating chamber. Material sputtered from the target is thus deposited on the substrate. The sputtering process is well know in the art and is described, for example, in: THIN FILM PROCESSES, edited by Vossen and Kern, Academic Press Inc. 1978.
We have found that when the target is an antimony-tin-indium alloy, electrical arcing sometimes occurs during the deposition process. This arcing causes many problems. If the arc terminates at the substrate to be coated, a very serious defect in the coating occurs. For example, the arc can cause localized crystallization of the amorphous layer and, in extreme cases, ablation of the forming layer and damage to the substrate. In any event, an arc disrupts the even flow of current from the power supply and the result is non-uniform coating. There is no suggestion of a solution to this problem in the art of which we are aware.